Digital systems often require digital-to-analog (D/A) converters and analog-to-digital (A/D) converters to communicate with analog components. For example, the speed of a DC motor may be controlled by an analog voltage level or current. Previous patents related to D/A converters include U.S. Pat. Nos. 3,967,272 to Sandford, Jr.; 4,410,879 to Gumm; 4,584,568 to Zomorrodi; 4,616,212 to Law; 4,618,852 to Kelley; 4,665,380 to ; 4,667,179 to Law; 4,973,979 to Ikeda; 4,998,108 to Ginthner; and 5,489,904 to Hadidi. Articles related to D/A converters include "All-MOS charge Redistribution Analog-to-Digital Conversion Techniques--Part 1" by McCreary, James L. and Gray, Paul R. in IEEE Journal of Solid State Circuits, vol. Sc-10, pp. 371-79, Dec 1975; and "Integrated PCM Codec" by Ohri, Kul B. and Callahan, Michael J. in IEEE Journal of Solid-State Circuits, vol. SC-14, pp. 36-46, Feb. 1979. The above citations are all incorporated herein in whole by reference.
Herein, voltage is abbreviated V, charge is abbreviated Q, current is abbreviated C, digital is abbreviated D, and analog is abbreviated A. Other abbreviations will be introduced below in the description to reduce unnecessary repetition.